Can we recover valuable chemicals from sewage? 2

Can we recover valuable chemicals from sewage?


Will Tarpeh hopes to make the word “wastewater” obsolete.

An assistant professor of chemical engineering, with a courtesy appointment in civil and environmental engineering, Tarpeh is studying how to extract ammonia, sulfur and other chemicals from sewage. His immediate goal is to turn these waste chemicals into fertilizers, disinfectants and other products, while leaving the water safer and cleaner. We sat down with Tarpeh to ask how his current research could lead to industrial-scale processes that might one day even be used to perform chemical cleanups of polluted waterways.

What is your research vision?

We try to recover valuable chemical products from wastewater. We’re starting with raw sewage but we envision moving ahead to clean up polluted ponds and waterways. We are engineering new chemical processes to extract valuable chemicals like sulfur, nitrogen and phosphorous from water, and then use these to create valuable products like fertilizers, disinfectants and more. It’s recycling, but for water.

The ammonia in urine, for instance, can be used to make ammonium sulfate, a common fertilizer. We’re also looking into using ammonia extracted from wastewater to make disinfectants, such as for household cleaners. We’re also starting to look at being able to recover the sort of chemicals used in large-scale industrial processes, like hydrochloric acid, sodium hydroxide and sulfuric acid.

How did you get started on this path?

As an undergraduate at Stanford I majored in chemical engineering but I was also interested in African studies. I saw sanitation as a global challenge that we were nowhere close to solving. I started to think of how I, as an engineer, could contribute. So I looked for ways to re-imagine the toilet, not just as a structure for gathering waste that would have to be disposed of, but as a collection center for raw materials that we could reuse to produce valuable products.

How do you extract chemicals out of wastewater?

We do so in a couple ways. Let’s start with electrochemistry. We hook up batteries to electrodes and run electrical current through the wastewater to attract certain chemicals we want to retrieve. For instance, ammonia is positively charged. We can get ammonia to move through the water toward a negatively charged electrode. But if we just put the electrodes into the wastewater, the negative pole would attract all the positively charged chemicals – ammonia, sodium, potassium and others. So we protect the electrode with a series of membranes that, in this case, only allow ammonia to filter through. But the chemical engineering gets complicated. We have to use more than one membrane to extract ammonia, which has this unique property that allows it to become a gas. So, we first convert the ammonia to a gas and then filter this gas through a second membrane. This enables us to recover 94 percent of the ammonia and very little of anything else. We are now fine-tuning the process. It works, and that’s great, but we want to know why it works and how we can make it better.


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